Season the beef with salt and pepper. Heat the oil in a heavy pot and sear the beef in batches, then remove. Sear the fresh mushrooms with a little salt and remove.
Brown the onions in the same pot. Add the garlic and cook until golden, then cook the tomato paste until brick red.
Add the pierced Persian lime and the stock. Return the beef and mushrooms. Cover and braise at 325F / 165C for 2 1/2 to 3 1/2 hours until fork tender. Taste after 1 hour and remove the lime once its flavor is pronounced.
Boil the potatoes in salted water until tender. Drain and mash with the 30 g butter. Cool completely, then mix in the Gruyere.
For each boureka, layer 2 phyllo sheets, brushing each with melted butter (keep unused sheets covered). Shape one-sixth of the filling into a 4-inch square in the centre. Fold the sides, top and bottom over the filling, buttering any dry pastry, and place seam side down.
Once the beef is cooked, raise the oven to 400F / 200C. Brush the parcels with melted butter and sprinkle immediately with sesame and nigella seeds. Bake 25 to 30 minutes until golden and crisp.
When the beef is tender, transfer the beef and mushrooms to a separate pan and keep warm. Strain the braising liquid through a fine-mesh strainer, discarding the onions, garlic and black lime. Chill the jus until the fat solidifies, remove the fat, then reduce the jus until glossy.
Top each boureka with beef, mushrooms and a little jus. Finish with chopped chives and serve immediately.
From chef_shimi_aaron on Instagram. Chuck roast slow-braised with oyster and shiitake mushrooms, onions, garlic and a Persian black lime, then served over crisp phyllo bourekas stuffed with Yukon Gold potato mash and Gruyere. The braising jus is strained, defatted and reduced glossy to spoon over. Makes 6. A project cook, not a weeknight.
